This invention relates to an inverter apparatus comprising a circuit which interrupts the delivery of drive signals to an inverter so as to protect the constituent elements of the inverter from overcurrents.
FIG. 2 shows a prior-art inverter apparatus of this type in terms of a circuit diagram. Referring to the figure, letter E indicates a D.C. power source, and numeral 1 designates a capacitor for suppressing a D.C. voltage ripple. Numerals 2-5 designate transistors constituting an inverter INV, and numerals 6-9 free-wheel diodes. Numeral 10 denotes an output transformer, numeral 11 a reactor for improving a waveform, numeral 12 an output filter capacitor, and numeral 13 an A.C. load. Numeral 14 denotes a current detector CT, numeral 15 an inverter overcurrent detecting circuit, numeral 16 a one-shot multivibrator, numeral 17 a base signal generating circuit, numeral 18 an interlock circuit, and numeral 19 a base drive circuit, and a control circuit for the inverter is constructed of the portions 14-19.
Next, the operation of this inverter apparatus will be explained.
When the output current of the inverter INV, namely, a current flowing through the current transformer 14 is not greater than an allowable value, the inverter overcurrent detecting circuit 15 is not actuated, and a base signal produced by the base signal generating circuit 17 is supplied to the base drive circuit 19. Upon receiving base drive signals from the base drive circuit 19, the transistors 2-5 perform on/off operations in a predetermined sequence and feed a single-phase alternating current to the A.C. load 13. Since the single-phase A.C. output is a rectangular wave alternating current, it is passed through the output transformer 10 and is thereafter subjected to waveform shaping by means of the reactor 11 as well as the output filter capacitor 12. Thus, the shaped output is fed to the A.C. load 13.
Here, when the load 13 falls into an overload state, the inverter overcurrent detecting circuit 15 is actuated to supply the one-shot multivibrator 16 with an overcurrent detection signal (trigger pulse). Then, the one-shot multivibrator 16 delivers a pulse signal of predetermined time width to the interlock circuit 18. Therefore, while this pulse signal is delivered, the interlock circuit 18 cuts off the supply of the base signal produced by the base signal generating circuit 17, to the base drive circuit 19. Incidentally, the time width of the pulse signal which the one-shot multivibrator 16 produces is previously set in consideration of the characteristics of the A.C. load 13, etc.
In this way, the transistors being the constituent elements of the inverter and the free-wheel diodes connected in parallel with them are protected from overcurrents. In the inverter apparatus of this type having the output filter capacitor 12, however, when the base signal is cut off, a case shown in FIG. 3 where the output filter capacitor 12 stores charges in an illustrated polarity occurs to give rise to a mode in which current circulates through the free-wheel diodes as indicated by arrows in solid lines. When, under such a state, the interlock operation of the interlock circuit 18 is released to supply the transistors of the inverter INV with the base drive signals, the stored charges of the output filter capacitor 12 incur a mode in which current flows along the transformer 10--the transistor 3 turned `on`--the free-wheel diode 9 as indicated by arrows of solid lines in FIG. 4, that is, a reactor short-circuit mode. This has posed the problem that a situation leading to the breakdown of the transistor or/and the diode ascribable to an overcurrent takes place.